Photosensitive materials which can be used in preparing either non-relief or relief images are well known in the art. These materials generally comprise a polymeric binder which serves to hold the material together and may or may not enter into a photoinitiated chemical reaction, a photoinitiator or photoinitiator system and, as needed, other components which are capable of reacting with the photoinitiator to produce a change in the physical properties of the material. These components are usually layered on a substrate or support before exposing to actinic radiation through an image-bearing transparency to produce either a positive or negative latent image. The differences in the properties of the exposed and unexposed areas of the photosensitive material, e.g., differences in solubility or swellability, differences in the softening point, differences in tackiness, differences in adhesion, differences in permeability for processing solutions, etc., are used as the basis for developing the image. Depending on the intended end use the developed image may be visible due to color differences, relief height differences, or combinations of the two. For some uses it is not necessary that the developed image be visibly discernible.
A photosensitive element can be made by solvent coating or melt extruding and laminating the photosensitive layer onto the support. Solution coating is commonly used to make photoresists, proofing media and lithographic printing plates. Melt extrusion is frequently used to produce flexographic printing plates. Each of these processes suffers from some disadvantages.
One problem with solvent coating is that it is often difficult to find solvents in which all the ingredients in the photosensitive layer have sufficient solubility so that a smooth transparent coating can be formed. This is particularly true for systems which are developable in aqueous solutions. Furthermore, non-aqueous coating solvents can pose safety and environmental hazards.
In melt extrusion, processing of the photosensitive layer at high temperatures diminishes both sensitivity and photospeed. Thus, the temperature must be kept sufficiently low to prevent degradation and thermally initiated reactions from taking place during processing. Polymeric binders which can be used to formulate a photosensitive composition, are limited to those having low melt temperatures and low melt viscosities in order to maintain sufficiently low processing temperatures. Processing latitude can often be gained by incorporating thermal stabilizers, but these stabilizers often decrease the photosensitivity of the resulting elements.
The various components must be well dispersed within the polymeric binder in order to prevent light scatter from non-homogeneities. Light scatter, in turn, can reduce optical resolution of these elements. In the melt extrusion process, it is often difficult to obtain excellent distributive mixing of the various components at low processing temperatures.